Question regarding Holley Carb (& vac advance)
08-14-2005, 03:35 PM
#1
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Question regarding Holley Carb (& vac advance)
I'm trying to improve my knowledge and understanding of
timing and vacuum advance, and I have been doing some
reading on the subject (including all documents by Lars).
However, I can't find much info on my Holley 4175. Anyone
know which of the ports (numbered in the pic) is manifold
vacuum, venturi vacuum, or ported vac. I have checked
with my vacuum gauge and ports 1 & 3 are 0" at idle.
#1 keeps increasing with revs, however #3 seems to top
out. #2 is about 17" (I think) at idle.
timing and vacuum advance, and I have been doing some
reading on the subject (including all documents by Lars).
However, I can't find much info on my Holley 4175. Anyone
know which of the ports (numbered in the pic) is manifold
vacuum, venturi vacuum, or ported vac. I have checked
with my vacuum gauge and ports 1 & 3 are 0" at idle.
#1 keeps increasing with revs, however #3 seems to top
out. #2 is about 17" (I think) at idle.
08-14-2005, 05:16 PM
#2
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My 4175 not quite the same.
Are u ready for this? 
Ok here's what i found.
Ur #1 is a ported vacuum source for ur dist vac can.
Ur #2 location has 2 ports on mine but only 1 port is direct manifold vac (the 1 on the inside) and used for the choke pull-off diaphram can (which looks to b correct h/u on urs). The outside or choke side port at ur #2 location is a ported vac for the EVC cannister.
Finnally ur #3 port isn't even drilled open on mine - just a flush casting there. But from ur pix that #3 looks like a direct manifold vac and now goes to the dist can.
Well good luck and BTW whats an LT4 doing in an early '71? Thought the LT4 was a '90s vette thing? cardo0
Correction here. If u read 0" vac on #3 at idle then that ain't no manifold vac. Sorry, can't tell u what animal lives in there. But my guess would be for the EVC h/u. BTW #1 may increase with revs but should decrease with load. 
cardo0
Ok here's what i found.Ur #1 is a ported vacuum source for ur dist vac can.
Ur #2 location has 2 ports on mine but only 1 port is direct manifold vac (the 1 on the inside) and used for the choke pull-off diaphram can (which looks to b correct h/u on urs). The outside or choke side port at ur #2 location is a ported vac for the EVC cannister.
Finnally ur #3 port isn't even drilled open on mine - just a flush casting there. But from ur pix that #3 looks like a direct manifold vac and now goes to the dist can.
Well good luck and BTW whats an LT4 doing in an early '71? Thought the LT4 was a '90s vette thing? cardo0
Correction here. If u read 0" vac on #3 at idle then that ain't no manifold vac. Sorry, can't tell u what animal lives in there.
But my guess would be for the EVC h/u. BTW #1 may increase with revs but should decrease with load. cardo0
Last edited by cardo0; 08-14-2005 at 05:25 PM. Reason: correction on h/u
08-14-2005, 06:06 PM
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cardo0, thanks for the info. I didn't know that the two ports
at the #2 position were different.
My mechanic had connected the vac advance to manifold
vacuum (same one that the choke is connected to). Somehow
that didn't make sense to me. Maybe time to find a new shop?
I have checked locations 1 and 3 again, and they are measuring
the same, so I guess either 1 or 3 should be fine for the advance.
As for the name...I no longer own an LT4 but didn't want to change
it (the name that is).
at the #2 position were different.
My mechanic had connected the vac advance to manifold
vacuum (same one that the choke is connected to). Somehow
that didn't make sense to me. Maybe time to find a new shop?
I have checked locations 1 and 3 again, and they are measuring
the same, so I guess either 1 or 3 should be fine for the advance.
As for the name...I no longer own an LT4 but didn't want to change
it (the name that is).
Last edited by LT4Coupe; 08-14-2005 at 06:52 PM.
08-14-2005, 10:41 PM
#4
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Nothing wrong with manifold vacuum on the dist unless u need to pass smog.
Originally Posted by LT4Coupe
cardo0, thanks for the info. I didn't know that the two ports
at the #2 position were different.
My mechanic had connected the vac advance to manifold
vacuum (same one that the choke is connected to). Somehow
that didn't make sense to me. Maybe time to find a new shop?
I have checked locations 1 and 3 again, and they are measuring
the same, so I guess either 1 or 3 should be fine for the advance.
As for the name...I no longer own an LT4 but didn't want to change
it (the name that is).
at the #2 position were different.
My mechanic had connected the vac advance to manifold
vacuum (same one that the choke is connected to). Somehow
that didn't make sense to me. Maybe time to find a new shop?
I have checked locations 1 and 3 again, and they are measuring
the same, so I guess either 1 or 3 should be fine for the advance.
As for the name...I no longer own an LT4 but didn't want to change
it (the name that is).
Only change u really need is move the dist vac line off #3 and onto #1 (ported vac) or tee into #2 (for manifold vac).
I'd say that shop did the right thing.
BTW 17" vac at idle is great.
And i think the Holley speadbore makes a great carb to tune and learn with - much simpler than a Qjet. I never hear complaints 'bout the Holley speadbores but plenty with the Qjets. The Qjet may be a little more responsive but its a whole lot more complicated. cardo0